Since you have seen black specks on the vascular system of the vine, it is most likely one of the symptoms of ESCA disease. Foliar and berry symptoms are the vine’s reactions to toxin production by the ESCA pathogen, and thus, it may take some time to develop.
(Photo credit: Rhonda Smith)
If the discoloration is brown, rather than black, the vine might be suffering from Botryosphaeria pathogens. Infected vines may not show striking foliar or shoot symptoms.
(Photo credit: Mizuho Nita)
Also, cankers can be caused by abiotic factors.